Device comprising a cavity resonator



July 5, 1960 BELJERS ETAL 2,944,232

DEVICE COMPRISING A CAVITY RESONATOR Filed April 19, 1954 I u l l r I l l l --m m 1 kavtonbq bras m m m Q N a A EN JKN/ L A D A T BNWM O SW TEIE Ne... EN w o GR ou 9 HKA Patented July 5, 1960 DEVICE COMPRISING A CAVITY RESONATOR Hugo Gerrit Beljers, Kornelis Swier Run], and Anton Eduard Pannenborg, Emmasingel, Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Apr. 19, 1954, s t. No. 424,134

the axis 3 and partly parallel thereto. Such mode of Claims priority, application Netherlands Apr. 29, 1953 i i 4 Claims. (Cl. 333-73) This invention relates to devices comprising a cavity polarising field is obtained on the ground of quite dif-. .ferent causes by a particular choice of the. mode of oscillation of the resonator. Variation in the polarising field in this device likewise results in amplitude modulation of the highfrequency oscillation, but this, may becompletely free from the phase modulationwhichwas still noticeable in the device previously suggested, whilst furthermore the modulating. process remains active for arbitrarily high frequencies of the high-frequency oscillation in the resonator.

According to the invention, a cavity resonator is used which is symmetrical across at least two. different diameters thereof, such as a cavity having a circular or square cross-sectionalshape, and a ferromagnetic body within this cavity is magnetically polarized at a value so as to cause thecavity, when electrically excited for producing an electrical transverse TE mode of oscillation, to have two different resonance frequencies for the two circularlypolarized components of the transverse oscillation; By varying the strength of the magnetic polarization, the frequency spacing between the two different resonance frequencies can be varied. i

In order that the invention may be readily carried into eifect, it will now be described with reference to the accompanying drawing, given by way of example, in which Fig. 1 is an axial section of a device according to the invention.

Fig. 2 shows the variation of the electric field strength in the resonator with reference to a cross-section.

Fig. 3 shows a variant of the device of Fig. 1.

Fig. 4 shows characteristics of resonance curves obtained with such devices.

Fig. 5 shows in greater detail an embodiment which serves more particularly for amplitude modulation of a high-frequency oscillation.

Referring now to Fig. 1, reference numeral 1 indicates a cylindrical cavity resonator which is excited by way of a coupling loop 2 in transverse high-frequency oscillation of a wavelength, for example, of the order of 1 cm., that is to say that the electrical field passes from one half of the wall of the cylinder to the other half (see Fig. 2), so that the electrical field-strength component E of the high-frequency oscillation is at right angles to the MlS of rotation 3 of the cylindrical cavity, whereas the magnetic field-strength component H, as may be seen from Fig. 1, is found to extend partly at right angles to oscillation is indicated by the symbol TE Provided in the bottom of the resonator 1 is a ferromagnetic body 4, preferably made of material having a large depth of penetration for the high-frequency oscillation, for example, ferrite, which body is subject to the action of a polarising magnetic field H produced by a magnet 5. Ifgdesired, the body 4 may be replaced by a tubular ferroinagneticybody 4 provided concentrically withthe axis of rotation 3 or combined with this body (see Fig. 3), in which event comparatively small fields 1-1 are sufficient.

Fig. 4 shows resonance curves of such a cavity resonator for ditferent values of the polarising field. The term resonance curve is to be understood here to mean the amplitude of the high-frequency oscillation generatedin the resonator as a function of the frequency f with a constant excitation oscillation at the coupling loop 2 and measured at dilferent values of the strength H of the polarising field.

.The curve a, which applies to a comparatively Weak polarising field, is similar to the resonance curve of a bandpass filter coupled critically, the curves b and c, which correspond to increasing values of the field strength H being similar to those of bandpass filters coupled overcritic ally. i

This behaviour may be explained as follows: The transverse high-frequency oscillation in the cavity resonator 1 may be decomposed into two circularly-polarised components of opposite senses of rotation about the axis of i rotation 3. Said components can build up themselves as independent oscillations in the resonator 1 only if the.

resonator has natural frequencies which substantially coincide inat least two different directions, for example, at right angles to one another,;the plane of which is parallel. to. the electrical field strength E, that is to say if the resonator has a cross-sectional area which is circular or, for example, square in shape. If, however, the resonator would have, for example, a rectangular crosssection which thus corresponds to two difierent natural frequencies of the resonator in two relatively perpendicular directions,.the said decomposition into circularlypolarised components has no real physical importance. If the field H is equal to zero, the ferromagnetic body 4 exerts the same influence upon the said two components, resulting in a resonance curve corresponding to that of a single circuit. However, if the field H diifers from zero, the susceptibility and the propagation constant of the body 4 become difierent for the two circularlypolarised oscillations, as is known, as a result of the Faraday eifect, so that the resonator also has different resonance frequencies for the said two oscillations, from which ensues the group of characteristic lines shown in Fig. 4.

If a band-pass filter characteristic is desired having a constant band-width, for example, of the character of the curve a or b in Fig. 4, the magnet 5 may be in the form of a permanent magnet, for example, of a small disc of ferroxdure (not shown) which is pressed against the body 4. If, however, a variable band-width is desired, the magnetic field may be varied, for example, between the values of the polarisation field strength H corresponding to the curves a and b in Fig. 4.

If the frequency of the excitation oscillation at the coupling loop 2 coincides with the resonance frequency f of the resonator 1 with ferromagnetic body 4 nonpolarised, the amplitude with which the oscillation is built up in the resonator 1 decreases if the strength H of the polarisation field increases and more particularly if the body 4 has the flat shape shown in Fig. 1, the said amplitude is substantially linearly dependent upon the field strength H However, since the phases of the two a resonator 1 having a'ferromagnetic body 4-and a magnet. 5 similar to-that shown ilTFlg; 1'. The resonator 1 is coupled hy way of a coupling hole 7 provided'conce'ntri'cally with the antis- 3 to a wave lineS', 9 of, for examp lc,-. rectangular cross-section tdwhich a high-frequency oscillation having a frequency-substantially equal to f issuppliecl by awave genera-tor for example, a klystron generator, the resonator l bri'nging about a more or less strong reflection of the saidoscillation as a function of the field H so that both the oscillation reflected inthe line 8 and the oscillation passed to the line 9 are modulated in amplitude. v y

The described phenomena maybe found up to arbitrarily: high frequencies corresponding to the natural freqnency of'the resonator. However, at lower frequencies, for example, up to about 10,000 mc'./s., they may become less distinct, since in this case the resonator 1 is also liable to be damped and de'tuned in accordance with the strength H}, of the polarisation field as a result of the inner structure oftheferromagnetic material of the body 4. However, this effect is found to disappear if the saturation manetisatione124 of the ferromagnetic material, 1nultiplied"by the gyromagnetic constant (:23 ms./s'/ Gauss), remains smaller than the frequency of the'transverse high-frequency oscillation;

What is claimed is: a v

"1 A- cavity resonator device comprising a cavity, a ferromagnetic material which exhibits the gyromagnetic effect atrnicrowa-ve frequencies being positionedwith said cavity, magnetic field means for magnetically polarizing sa-idfferromagnetic body in a direction parallel to theaxis of said cavity, said cavity having symmetrical dimensions in at least two directions in a plane normal to said axis, so that said resonator device has at least two substantially equal natural frequencies of resonance corresponding to said two' directions; and means for producing within said cavity a transverse highfrequency u ation having an electric field normal to the of sa jd'cavity, said magnetic polarization of' said ferromagnetic body being sufiicient to cause said cavity resonator device to-have two difierent simultaneous reso-- ponents' of said transverse oscillation.

' 2. A device as claimed inclaim- 1, including, means for varying the value of said polarizing field, thereby to amplitude-modulate said high-frequency oscillation sub stantia-lly without causing phase-modulation thereof.

3. A cavity resonator device comprising a cavity, aferromagnetic material which exhibits the gyromagnetic nant frequencies for the-two circularly polarized comfrequencies for the two ci'rcularlypolarized components effect at microwave frequencies being positioned with said cavity, magnetic field means for magnetically polarizing said ferromagnetic body in a direction parallel to the axis of 'said cavity, said cavity having symmetrical dimensions in at least two directions in a plane normal to said axis, so that said resonator device has at least two substantially equal natural frequencies of resonance corresponding to said two directions, and means for pro: ducingwithin said cavity a transverse high frequency oscillation having an electric field normal to the :axis of said cavity, said magnetic polarization of said ferromagnetic body being suflicient to cause said cavity resonator device to have two different simultaneous resonant of said transverse oscillation, and means for adjusting said magnetic polarization to a value at which the resonance characteristic curve of said cavity is that of a bandpass filter.

4. A cavity resonator device comprising a cavity, a ferromagnetic material which exhibitsthe gyromagnetic effectat microwave frequencies being positioned with said cavity, magnetic field. means for magnetically polarizingsaid ferromagnetic body in a direction parallel to the axis of said cavity-said cavity having synnnetrical dimensions'in at least two directions in a plane normal to. said axis, so that said resonator device has at least two substantially equal natural frequencies of resonance corresponding to said two directions, and means for producing within said cavity a transverse high frequency oscillationhaving an electric field normal to the axis of said cavity; saidmagnetic polarization of said ferro-- magnetic-body being suficient' to cause said cavity resonatordevice to'have twodifierent simultaneousresonant frequencies for the twocircularly polarized' components of saidtransverse oscillation; and means 'for adjusting. I

said magnetic pol'arization'to Ia value at which the. reso 'nance characteristic curve of said cavity'resonator has two dilferent resonance peaks.

References Cited in the' iil'e of this patent UNITED :STATES PATENTS 7 2,645,758. was de iindt. t July 14,1953

2,784,378 Yager l Mar. 5 1957,

2,798,295 V. Hogan July 2, 1957 FOREIGN r TnNTs I 674,874"

Philips Technical Review; vol. 11, No. 11; May 1950,

pages 3 13-322. 

